Gated Alarm

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Sometimes the need
arises for a simple, gated, pulsed alarm. The circuit shown here employs
just four components and a piezo sounder and is unlikely to be out-done
for simplicity. While it does not offer the most powerful output, it is
likely to be adequate for many applications. A dual CMOS
timer IC type 7556 is used for the purpose, with each of its two halves
being wired as a simple astable oscillator (a standard 556 IC will not
work in this circuit, nor will two standard 555’s). Note that the
CMOS7556 is supplied by many different manufacturers, each using their
own type code prefix and suffix. The relevant Texas Instruments product,
for instance, will be marked ‘TLC556CN’.

The circuit configuration used here is seldom seen, due probably to
the inability of this oscillator to be more than lightly loaded without
disturbing the timing. However, it is particularly useful for high
impedance logic inputs, since it provides a simple means of obtaining a
square wave with 1:1 mark-space ratio, which the ‘orthodox’
configuration does not so easily provide. IC1.A is a slow oscillator
which is enabled when reset pin 4 is taken High, and inhibited when it
is taken Low. Output pin 5 of IC1.A pulses audio oscillator IC1.B, which
is similarly enabled when reset pin 10 is taken High, and inhibited
when it is taken Low.

Gated Alarm Circuit

Gated Alarm Circuit Diagram

In order to simplify oscillator IC1.B, piezo sounder X1 doubles as
both timing capacitor and sounder. This is possible because a passive
piezo sounder typically has a capacitance of a few tens of nanofarads,
although this may vary greatly. As the capacitor-sounder charges and
discharges, so a tone is emitted. The value of resistor R2 needs to be
selected so as to find the resonant frequency of the piezo sounder, and
with this its maximum volume. The circuit will operate off any supply
voltage between 2 V and 18 V. A satisfactory output will be obtained at
relatively high supply voltages, but do not exceed 18 V.

Author: Rev. Thomas Scarborough – Copyright: Elektor July-August 2004